Prior to the commercial introduction of metallocene type catalysts, the heterogeneity of catalysts such as chrome based catalyst and the Ziegler Natta catalyst contributed far more to the heterogeneity of the resulting polymer than the effects of the support. With the introduction of metallocene type catalysts (single site catalysts), the heterogeneity of the support may significantly contribute to the heterogeneity of the resulting polymer. Depending on the catalyst, this may lead to an undesirable amount of a higher molecular weight fraction (e.g. a significant second peak at the right hand side (high elution temperature) of a TREF curve).
U.S. Pat. No. 4,923,833 issued May 8, 1990 to Kioka et al., assigned to Mitsui Petrochemical Industries, Ltd. teaches a number of methods to prepare a metallocene catalyst in the presence of an activator without a support such as silica. The second method described from line 62 of column 9 through line 45 of column 10 teaches forming a solution of MAO and a metallocene and adding to it a non-solvent for the MAO to precipitate the MAO and metallocene. The patent does not disclose catalysts containing a phosphinimine ligand nor does it disclosure the use of fluorinated hydrocarbons as a non solvent for the MAO and the catalyst.
There are a number of related patents in the name of Denifl et al, assigned to Borealis Technology Oy that disclose a method to prepare a catalyst without an external support (self supported catalyst). These patents are related to or derived from PCT/EPO02/14461 (such as U.S. Pat. No. 7,341,971 issued Mar. 11, 2008). The thrust of the disclosure is to metallocene type catalysts and there does not appear to be any disclosure of a catalyst containing a phosphinimine ligand. The reference teaches that an organic solution of catalyst and activator together with a surfactant are added to a liquid under conditions at which the liquid is a non solvent for the solution of activator and catalyst (i.e. an emulsion is formed). The resulting emulsion may then be subjected to conditions which cause the organic solvent to become miscible in the liquid. This phase change causes the catalyst particles formerly dispersed in the organic solvent to be precipitated from the liquid. The particles may then be recovered. In a preferred embodiment of the patents the catalyst is prepolymerized prior to use in a commercial reactor.
The Borealis patents suggest the support can also have an adverse effect on the activity of the catalyst, on its polymerization behavior and on the properties of the end polymer (e.g. U.S. Pat. No. 7,341,971 Col. 1 lines.58-62). The disclosures of the Borealis patents focus more on the morphology of the polymer particles and very little is disclosed about the architecture of the resulting polymer.
Phosphinimine containing catalysts are also known in the patent literature. There are a number of patents in the names of Stephan and Brown among others assigned to NOVA Chemicals Corporation and NOVA Chemicals International S.A. relating to the use of transition metal complexes containing a cyclopentadienyl type ligand and a phosphinimine ligand for use as a catalyst in the polymerization of alpha olefins. In solution phase polymerization where such a catalyst is not supported the resulting polymer is bimodal having a component which is characterized in U.S. Pat. No. 6,984,695 as having a higher molecular weight and a high density. Most preferably this fraction is present in an amount from 2 to 10 weight % of the polymer.
When some phosphinimine containing catalysts, as described below, are used in supported form for example in a gas phase or slurry phase polymerization the high molecular weight/high density fraction may become excessive. Surprisingly, Applicant has found that by treating such catalyst in accordance with the process described in the Borealis patents the amount of the high molecular weight/high density fraction in the resulting polymer is significantly reduced.
The present invention seeks to provide a copolymer comprising from 80 to 99 weight % of ethylene and from 20 to 1 weight % of one or C4-8 alpha olefins, said copolymer having a density from 0.910 to 0.945 g/cc; a CDBI greater than 65% as measured using TREF; a polydispersity from 2.5 to 3.5, and a bimodal TREF curve, prepared by a gas phase or slurry polymerization in the presence of a self supported catalyst prepared in the presence of a catalyst containing a phosphinimine ligand and an activator, which catalyst is used without an external support, such as silica or alumina.